Imaging spectrometers are used for a wide variety of strategic, scientific and resource sensing applications. Typically, an imaging spectrometer system includes an objective (also called an imaging optical module) that forms a scene image at a slit and a spectral optical module, which receives and collimates the line field of view from the objective, disperses or separates the radiation as a function of wavelength, and images it onto a two-dimensional detector array. One dimension of the array contains spatial information, while the other contains spectral information. The spectral optical module usually takes the form of an image relay between the slit and a focal plane (where the detector array can be located) with a dispersing element, most commonly a prism or a diffraction grating, located at a pupil between the images.
Common imaging spectrometers include the Offner-Chrisp and Dyson optical forms. Reflective triplet imaging spectrometers are also used, particularly where it is desired to cover a very wide field of view (FOV). Examples of a reflective triplet imaging spectrometer are described in U.S. Pat. No. 5,260,767. It is generally desired that the size of the spectrometer, and the power needed to cool the spectrometer, especially for infrared applications, is minimized. This is especially true when the instrument is used on mobile platforms, such as aircraft or spacecraft. The size of the spectrometer is traditionally driven by the optical f/# (a faster instrument generally requires larger optics) and the field of view (a larger FOV also generally requires larger optics). The Dyson optical form is generally much smaller than the Offner-Chrisp optical form when fast (low) f/# s are required, but uses refractive optics (lenses) and therefore suffers from the limitations and disadvantages associated with those refractive optics, including chromatic aberration, limited spectral bandwidth, defocus with temperature change requiring compensation, potential high narcissus, and high cost associated with the complexity and expensive refractive materials. All-reflective optical designs, such as the Offner-Chrisp and reflective triplet, avoid these problems, but may not be able to achieve f/# s faster than about f/2, and are also generally larger than refractive optics.
The Offner-Chrisp imaging spectrometer is widely used, but has limited FOV with acceptable wavefront error and distortion for many imaging applications, and also cannot typically achieve optical speed faster than f/2. As an all-reflective design, it has wide spectral bandwidth, but much larger size than a reflective triplet design with the same f/#, FOV, and distortion. In addition, conventional Offner-Chrisp imaging spectrometers require a convex diffraction grating with tight spacing, and telecentric incident light, which necessitates cryo-cooled imaging optics (objective) for thermal infrared (e.g., in the long-wave infrared spectral band) systems. A monolithic Offner-Chrisp imaging spectrometer has been developed, as described in U.S. Pat. No. 7,697,137, which may achieve smaller size and faster f/# s than conventional Offner-Chrisp imaging spectrometers. Dyson imaging spectrometers are also commonly used, particularly where it is desired to achieve faster f/# s. The Dyson optical form is compact for a given f/#, FOV, and performance, but suffers from the limitation and disadvantages associated with refractive optics, as discussed above. In addition, Dyson imaging spectrometers require telecentric incident light, and therefore cryo-cooled imaging optics for thermal imaging applications, along with a concave diffraction grating.